[O II] Emission in the Diffuse Ionized Gas of NGC

accepted for publication in Astrophysical Journal Searching for additional heating | [O II] emission in the diffuse ionized gas of NGC 891,1 NGC 4631 and NGC 3079 B. Otte,2 R. J. Reynolds, J. S. Gallagher III2 Department of Astronomy, University of Wisconsin{Madison 475 North Charter Street, Madison, WI 53706 and A. M. N. Ferguson Kapteyn Astronomical Institute, University of Groningen P.O. Box 800, 9700 AV Groningen, The Netherlands ABSTRACT We present spectroscopic data of ionized gas in the disk–halo regions of three edge– on galaxies, NGC 891, NGC 4631 and NGC 3079, covering a wavelength range from [O II] λ3727Ato[S˚ II] λ6716.4A.˚ The inclusion of the [O II] emission provides new constraints on the properties of the diffuse ionized gas (DIG), in particular, the origin of the observed spatial variations in the line intensity ratios. We used three different methods to derive electron temperatures, abundances and ionization fractions along the slit. The increase in the [O II]/Hα line ratio towards the halo in all three galaxies requires an increase either in electron temperature or in oxygen abundance. Keeping the oxygen abundance constant yields the most reasonable results for temperature, abundances, and ionization fractions. Since a constant oxygen abundance seems to require an increase in temperature towards the halo, we conclude that gradients in the electron temperature play a significant role in the observed variations in the optical line ratios from extraplanar DIG in these three spiral galaxies. Subject headings: ISM: abundances — ISM: general — ISM: individual (NGC 891, NGC 3079, NGC 4631) — galaxies: abundances — galaxies: general — galaxies: individual (NGC 891, NGC 3079, NGC 4631) 1Based on observations made with the William Herschel Telescope operated on the island of La Palma by the Isaac Newton Group in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias. 2Visiting Astronomer, Kitt Peak National Observatory, National Optical Astronomy Observatories, which is operated by the Association of Universities for Research in Astronomy, Inc. (AURA) under cooperative agreement with the National Science Foundation. –2– 1. INTRODUCTION When examining ionized gas, it is common practice to distinguish between classical H II regions (Strömgren spheres around OB stars) and diffuse ionized gas (DIG), the gas outside the boundaries of the Strömgren spheres. While H II regions are created by photoionization, the ionization processes for the DIG are less well known. Many attempts have been made to explain the DIG by photoionization models (e.g. Domgörgen & Mathis 1994; Sokolowski 1992), while a few studies address the possibility of shock excitation (e.g. Sivan, Stasińska, & Lequeux 1986). In recent years, DIG was found not only in the disks of galaxies, but also far above the stellar disks in the halo of the Milky Way and in several edge–on galaxies at heights of more than 1 kpc (e.g. Reynolds 1985; Rand, Kulkarni, & Hester 1990). Questions therefore arise about where this extraplanar DIG (eDIG) comes from and how it is ionized. Dynamical models of galaxies like ‘galactic fountains’ (Shapiro & Field 1976) and ‘chimneys’ (Norman & Ikeuchi 1989) describe how gas can be transported from the disk into the halo. Super- nova explosions heating the gas in the disk and pushing it up into the halo are important for both the dynamics and the ionization of the gas in the halo. Due to the high velocities in this ejected gas, shocks can arise and ionize the gas far above the disk. The models of runaway O stars leaving the disk and moving into the halo (e.g. Gies 1987), leaking ionizing photons from the disk into the halo due to low density gas (Miller & Cox 1993; Dove & Shull 1994) as well as the theory of photons created by neutrino decay (Sciama 1990) are further attempts to explain the ionization of extraplanar DIG. Both Martin (1997) and Rand (1998) were able to explain the run of several of the observed emission line ratios from H II regions to the DIG with composite models. Their models consisted of photoionization and one additional ionization process (shock ionization, turbulent mixing layers). This additional process was needed to explain the rise in the [O III]/Hβ line ratio with increasing distance from the disk. However, even with these composite models it was not possible to explain the constant [S II]/[N II] line ratio, which was observed in NGC 891 (Rand 1998), as well as in the Milky Way (Haffner et al. 1999) and other galaxies (Otte & Dettmar 1999). These data led Haffner et al. (1999) to the conclusion that the electron temperature increases with increasing distance from the midplane of the galaxies. A rise in temperature can explain both the growing [O III]/Hβ ratio as well as the constant [S II]/[N II] ratio with increasing galactic altitude z without invoking an additional ionization | | mechanism at high z . Such a rise in electron temperature also should effect the [O II]/Hα line ratio. | | The [O II] λ3727A˚ emission line provides important additional information about the ionization and heating processes in the DIG because of its high excitation energy. Below we present the results of observations of [O II], [O III], Hβ,[NII], Hα,and[SII] emission from the eDIG of three edge– on galaxies, NGC 891, NGC 4631 and NGC 3079. These objects represent the first targets of a small sample of edge–on galaxies which have been chosen for their known eDIG emission. The analysis of the other galaxies in our sample is still in progress. The results obtained from NGC 891, –3– NGC 4631 and NGC 3079 provide some evidence for an increase in temperature with increasing height. Additional information is obtained about variations in the ionization state and chemical abundances within the gaseous halos. 2. OBSERVATIONS AND DATA REDUCTION The spectra of NGC 891 were obtained with the ISIS spectrograph at the William Herschel 4.2 m Telescope on La Palma, Canary Islands, in 1999 September 10 under photometric conditions and dark skies. Gratings R316R and R300B were used for the red and the blue arm, respectively. Blocking filter GG495 was used for the red arm. The slitwidth was 1”. The blue arm was read out in a 2x2 binned mode and yielded pixel scales of 000.40/pixel or 1.74A/pixel.˚ The covered wavelength range was from about 3600 A˚ to 5400 A.˚ The red arm was rebinned after reduction and calibration to match the spatial pixel scale of the blue arm. The wavelength dispersion for the red arm was 1.47A/pixel˚ with a wavelength range from about 5700 A˚ to 7200 A.˚ We combined two 30 min and one 20 min exposures in each arm. The slit position is the same as in Rand (1998) and shown in Figure 1. The spectra of NGC 4631 and NGC 3079 were obtained with the GoldCam spectrograph at the 2.1 m telescope on Kitt Peak, AZ, in 2000 February 29 – March 6. We used grating 9 with decker 4 and a slitwidth of 200.5. This yielded a pixel scale of 000. 80/pixel or 2.44 A/pixel,˚ respectively, and a wavelength range from about 3500 A˚ to 7400 A.˚ We used filter WG345 to remove possible overlaps between orders. Exposure times varied from 20 min to 30 min depending on the weather. For NGC 4631, we combined eleven spectra with a total integration time of 4.5 hours. For NGC 3079, we combined nine spectra with a total integration time of 3 hours and 20 minutes. Both slit positions are perpendicular to the plane of the galaxies and shown in Figures 2 and 3. During the observations of galaxies, we also took a few spectra of blank sky regions. The spectra were reduced using standard procedures in IRAF3. For sky subtraction, we sub- tracted differently scaled sky spectra from the red and the blue part of the galaxy spectra of NGC 4631 and NGC 3079. This was necessary for the spectra taken during the first night, which was partly cloudy. The sky spectra were smoothed along the slit by 15 pixels to increase the per– pixel signal–to–noise ratios. The resulting spectra are of very good quality, and, in particular, we appear to have successfully removed the sky lines that can be confused with [O II] emission. For sky subtraction in our NGC 891 spectra, we averaged over about 100 rows in the galaxy spectra which did not show any galactic emission and used this average spectrum as sky. The spectra were then calibrated in wavelength and corrected for distortion using standard star exposures at different positions along the slit. After the flux calibration, we combined the spectra at each slit 3IRAF is distributed by the National Optical Astronomy Observatories, which are operated by the Association of Universities for Research in Astronomy, Inc., under cooperative agreement with the National Science Foundation. –4– position by carefully examining the positions of emission lines in the wavelength direction and the position of flux features in the spatial direction. Individual spectra were shifted by integer pixels, if necessary, to overlap features and thus avoid broadening of lines and features during the combining procedure. The routine for cosmic ray removal during combining (averaging) of the spectra was insufficient. We therefore did not remove cosmic rays during image combining, but cleared the areas around emission lines of cosmic rays by hand later, before measuring emission lines. We only used galaxy spectra of comparable signal–to–noise for the combining procedure, that is we did not include spectra whose count rates were significantly reduced due to obscuration by clouds.

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